Just stumbled across this nifty free (and open source!) tool: The Pencil Project. Pencil lets you sketch out your applications quickly and export them as PNG images. It offers, among other things, the standard operating systems widgets (currently WinXP and GTK, collection expandable) and is built upon XUL and SVG. Pencil runs as a lightweight 400k Firefox 3 extension, but can also be run as a stand-alone application.